Do not add a colon chunk to the code completion of class inheritance access modifiers

With enabled CINDEXTEST_CODE_COMPLETE_PATTERNS env option (which enables
IncludeCodePatterns in completion options) code completion after colon
currently suggests access modifiers with 2 completion chunks which is
incorrect.

Example:
class A : <Cursor>B
{
}

Currently we get 'NotImplemented:{TypedText public}{Colon :} (40)'
but the correct line is just 'NotImplemented:{TypedText public} (40)'

The fix introduces more specific scope that occurs between ':' and '{'
It allows us to determine when we don't need to add ':' as a second
chunk to the public/protected/private access modifiers.

Patch by Ivan Donchevskii!

Differential Revision: https://reviews.llvm.org/D38618

llvm-svn: 316436